OBJECTIVEConditional gene targeting has been extensively used for in vivo analysis of gene function in β-cell biology. The objective of this study was to examine whether mouse transgenic Cre lines, used to mediate β-cell– or pancreas-specific recombination, also drive Cre expression in the brain.RESEARCH DESIGN AND METHODSTransgenic Cre lines driven by Ins1, Ins2, and Pdx1 promoters were bred to R26R reporter strains. Cre activity was assessed by β-galactosidase or yellow fluorescent protein expression in the pancreas and the brain. Endogenous Pdx1 gene expression was monitored using Pdx1tm1Cvw lacZ knock-in mice. Cre expression in β-cells and co-localization of Cre activity with orexin-expressing and leptin-responsive neurons within the brain was assessed by immunohistochemistry.RESULTSAll transgenic Cre lines examined that used the Ins2 promoter to drive Cre expression showed widespread Cre activity in the brain, whereas Cre lines that used Pdx1 promoter fragments showed more restricted Cre activity primarily within the hypothalamus. Immunohistochemical analysis of the hypothalamus from Tg(Pdx1-cre)89.1Dam mice revealed Cre activity in neurons expressing orexin and in neurons activated by leptin. Tg(Ins1-Cre/ERT)1Lphi mice were the only line that lacked Cre activity in the brain.CONCLUSIONSCre-mediated gene manipulation using transgenic lines that express Cre under the control of the Ins2 and Pdx1 promoters are likely to alter gene expression in nutrient-sensing neurons. Therefore, data arising from the use of these transgenic Cre lines must be interpreted carefully to assess whether the resultant phenotype is solely attributable to alterations in the islet β-cells.

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] i ) is a key signal in the initiation of insulin secretion from the pancreatic ␤-cell. This increase principally results from calcium influx through plasma membrane (PM) Ca 2ϩ channels, which open in response to secretagogues, primarily glucose. The metabolism of glucose through glycolysis and the tricarboxylic acid cycle leads to an increase in the cytoplasmic ATP-to-ADP (ATP/ADP) ratio. This causes closure of ATP-sensitive K ϩ (K ATP ) channels followed by depolarization of the ␤-cell membrane to the threshold potential where Ca 2ϩ channels open, initiating Ca 2ϩ influx (4). These events underlie glucose-induced electrical activity that, in pancreatic islets, consists of Ca 2ϩ -dependent action potentials.There is extensive literature describing ␤-cell electrical activity and its relationship to [Ca 2ϩ ] i in intact islets of Langerhans, isolated islet cells, and insulinoma cell lines. Most of the work has been carried out using mouse islets, with some studies using islets from rat, hamster, human, and other species.Mouse pancreatic ␤-cells exhibit complex and cyclic spike-burst activity in response to a rise in extracellular glucose concentration. The bursts consist of a depolarized phase of Ca 2ϩ -carrying action potentials alternating with a silent phase of repolarization, resulting in oscillations in intracellular Ca 2ϩ , which can drive pulses of insulin secretion (28, 37).The only stimulus required for a complex cyclic spike-burst activity and corresponding [Ca 2ϩ ] i oscillations in islets and ␤-cell clusters is elevation of glucose to levels above 5 and less than ϳ20 mM. Intermediate glucose concentrations induce two main types of oscillations in mouse pancreatic islets: fast, where the period ranges from 10 to 30 s, and slow, with periods of several minutes (37,54,83). Single mouse ␤-cells can also respond to glucose stimulation with regular oscillations (37).We have previously studied slow and fast [Ca 2ϩ ] i oscillations in islets in response to a variety of conditions (70, 73; unpublished observations). We have also previously reported that a stable, transgenically derived murine insulinoma cell line (␤TC3-neo) responds to glucose with slow, large amplitude [Ca 2ϩ ] i oscillations but only in the presence of 10-20 mM tetraethylammonium (TEA), a blocker of K ϩ channels (74). We have utilized this cell line to characterize glucose-stimulated oscillatory activity (74).However, the precise interpretation of previous results is limited due to the numerous channels and pumps in ␤-cells that work concurrently, and identification of physiologically slow variables that drive oscillations remains unclear. To clarify these complex experimental results, we used a mathematical modeling approach. Our goals, then, are twofold: to develop a model for ␤-cell ion homeostasis, including the bursts and [Ca 2ϩ ] i oscillations that can simulate cellular behavior, and to explain on this basis the experimental data for single cells and islets.Several mathematical approaches in the literature have provid...

SUMMARY Glucagon-like peptide-1 (GLP-1), an insulinotropic peptide released from the intestine after eating, is essential for normal glucose tolerance (GT). To determine whether this effect is mediated directly by GLP-1 receptors (GLP1R) on islet β-cells, we developed mice with β-cell specific knockdown of Glp1r. β-cell Glp1r knockdown mice had impaired GT after intraperitoneal (IP) glucose, and did not secrete insulin in response to IP or intravenous GLP-1. However, they had normal GT after oral glucose, a response that was impaired by a GLP1R antagonist. β-cell Glp1r knockdown mice had blunted responses to a GLP1R agonist, but intact glucose lowering with a DPP-4 inhibitor. Thus, in mice, β-cell Glp1r are required to respond to hyperglycemia and exogenous GLP-1, but other factors compensate for reduced GLP-1 action on the β-cell during meal ingestion. These results support a role for extra-islet GLP1R in oral glucose tolerance and paracrine regulation of β-cells by islet GLP-1.

The glucagon-like peptide-1 (GLP-1) receptor and the glucose-dependent insulinotropic polypeptide (GIP) receptor transduce nutrient-stimulated signals to control beta cell function. Although the GLP-1 receptor (GLP-1R) is a validated drug target for diabetes, the importance of the GIP receptor (GIPR) for the function of beta cells remains uncertain. We demonstrate that mice with selective ablation of GIPR in beta cells (MIP-Cre:Gipr(Flox/Flox); Gipr(-/-βCell)) exhibit lower levels of meal-stimulated insulin secretion, decreased expansion of adipose tissue mass and preservation of insulin sensitivity when compared to MIP-Cre controls. Beta cells from Gipr(-/-βCell) mice display greater sensitivity to apoptosis and markedly lower islet expression of T cell-specific transcription factor-1 (TCF1, encoded by Tcf7), a protein not previously characterized in beta cells. GIP, but not GLP-1, promotes beta cell Tcf7 expression via a cyclic adenosine monophosphate (cAMP)-independent and extracellular signal-regulated kinase (ERK)-dependent pathway. Tcf7 (in mice) or TCF7 (in humans) levels are lower in islets taken from diabetic mice and in humans with type 2 diabetes; knockdown of TCF7 in human and mouse islets impairs the cytoprotective responsiveness to GIP and enhances the magnitude of apoptotic injury, whereas restoring TCF1 levels in beta cells from Gipr(-/-βCell) mice lowers the number of apoptotic cells compared to that seen in MIP-Cre controls. Tcf7(-/-) mice show impaired insulin secretion, deterioration of glucose tolerance with either aging and/or high-fat feeding and increased sensitivity to beta cell injury relative to wild-type (WT) controls. Hence the GIPR-TCF1 axis represents a potential therapeutic target for preserving both the function and survival of vulnerable, diabetic beta cells.